hammer: Enable internal pull-up on UART RX line
hammer: Enable internal pull-up on UART RX line

Prevent spurious wake when servo is not connected.

BRANCH=none
BUG=b:36119938
TEST=Measure pull up value with a multimeter when servo is not
     connected: value around 30k when host in S0 and S3.

Change-Id: I8bd54e4dcfd5c65f1f0cd32ffd4a70cedb979cc2
Reviewed-on: https://chromium-review.googlesource.com/452652
Commit-Ready: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Tested-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec